Freeradius configuration cannot be stored on a mapped Docker volume

[Timothy Weaver]

When attempting to include a volume mapping to anything under either the
/etc/raddb or /etc/freeradius container folders, the container crashes on
startup. Despite the original ticket being closed as a Docker issue, the
behavior does not occur with non Freeradius containers.

For example, if the docker-compose.yml file includes:

    volumes:
      - ./freeradius:/etc/freeradius

or

    volumes:
      - ./raddb:/etc/raddb

This will not allow the container to start. Only a mapping to a specific
file will work, but is sub-optimal because then you cannot persist anything
in the mods-enabled folder.

While the documentation explicitly handles this persistence with the `COPY
raddb/ /etc/raddb/` line, it seems this is a workaround to supporting a
very standard way of handling it in the docker-compose.yml.